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Scheme 3 Proposed mechanism for the Tishchenko reaction using 3
and 4 as pre-catalysts. [Mg] = ‘Mg(guanidinate)’, where guanidinate =
[hpp]ꢀ (3) and [tbo]ꢀ (4).
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´
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24 These data are also consistent with an intimate mixture of
Mg(hpp)2(THF)n + MgCl2(THF)2–n (n = 0–2). No satisfactory
mass spectral data could be obtained; elemental analysis is also
consistent with an intimate mixture of bis-THF solvated MgBr2/
Mg(hpp)2.
25 Attempts to isolate and characterize the bis-guanidinate complex
directly from the reaction of MgBu2 with 2 equiv. hppH were
frustrated by the poor solubility of the product.
To probe the scope of the substrate generality, the reactions
of 3 (1 mol%) with pivaldehyde, cyclohexanecarbaldehyde,
isovaleraldehyde and acetaldehyde were examined. The dimeri-
zation of pivaldehyde was slower than for benzaldehyde (10%
yield of the ester after 10 min, TOF = 60 hꢀ1), with an 83%
yield after 24 h and no observable side-products by NMR
spectroscopy. Dimerization also occurred with the enolizable
substrate cyclohexanecarbaldehyde. The catalysis was initially
more rapid, with a 56% yield of the ester after 10 min (TOF =
336 hꢀ1). However, only a 75% yield of ester is produced after
24 h with almost full consumption of the aldehyde (B2%
remaining), indicating the presence of side reactions. In contrast,
the reactions of 3 with isovaleraldehyde and acetaldehyde gave
complex mixtures of oligomeric species.
In summary, we have developed a viable synthetic route
to heteroleptic magnesium compounds incorporating bicyclic
guanidinate ligands and demonstrated their activity as
pre-catalysts in the Tishchenko reaction.
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Notes and references
z Expansion of the nomenclature used to describe guanidinates that
bridge more than one metal centre is given in the ESI.w
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30 The side-product in this case is PhC(O)N{SiMe3}2. Evidence for
the formation of this species came from GCMS analysis of the final
reaction mixture, in which a peak for the hydrolysis product
PhC(O)NH{SiMe3} was observed.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4995–4997 4997